Semiconductor optical motulator

ABSTRACT

Semiconductor optical modulator devices exhibiting improved chirp characteristics are constructed from a Mach-Zehnder structure having resonators positioned within each arm and a number of phases shifters positioned within the arms, and/or resonators.

FIELD OF THE INVENTION

This invention relates generally to the field of optical communicationsand in particular to optical modulators exhibiting improved chirpcharacteristics.

BACKGROUND OF THE INVENTION

Optical modulators constructed from Si or other semiconductor materialsmay impart certain phase changes or “chirp” to optical signals uponwhich they operate. Unfortunately, chirping degrades certaintransmission qualities of modulated light.

In addition, Si modulators exhibit a waveguide loss that is dependentupon refractive index change. Modulators exhibiting suchindex-change-dependent waveguide loss are unsuitable for use with manymodulation formats that are used in advanced optical communicationssystems.

In an article entitled “Linearized Mach-Zehnder Intensity Modulator”authored by X. Xie, J. Khurgin and J. Kang which appeared in IEEEPhotonics Technology Letters, Vol. 15, No. 4, in April 2003, the authorsdescribe an optical modulator in which certain deficiencies wereovercome, but not the undesirable chirping.

SUMMARY OF THE INVENTION

The problem of chirping in a semiconductor optical modulator issubstantially overcome in accordance with the principles of theinvention, by employing resonators in each arm of a Mach-Zehnder devicewhich is made part of the modulator. In a preferred embodiment, theresonators are ring resonators and the Mach-Zehnder device is driven ina push-pull configuration.

In accordance with an aspect of the present invention, each of theresonators exhibits a particular resonance that substantially overlapsone another when the modulator is turned off and have less overlap whenthe modulator is turned on. In sharp contrast to prior artconfigurations, this configuration advantageously reduces any requiredoptical index changes which, in turn, reduces changes in propagationlosses experienced in, for example, carrier injected silicon waveguidestructures thereby minimizing the chirp.

In accordance with yet another aspect of the invention, an opticalmodulator so constructed may advantageously produce differential phaseshift keying (DPSK) and higher order advanced modulation formats withminimal chirp.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1A is a schematic of a prior-art optical Mach-Zehnder modulator;

FIG. 1B is a schematic of an optical Mach-Zehnder modulator according tothe present invention;

FIG. 2 is a graph of Transmittance vs. Frequency for a modulator such asthat of FIG. 1, depicting the resonance offset from light to bemodulated for each of the arms;

FIG. 3 is a graph of Transmittance vs. Frequency for a modulator such asthat of FIG. 1, showing an “off” state of the modulator;

FIG. 4 is a graph of Phase vs. Frequency depicting the π phasedifference between the lower arm and the upper arm of the modulator ofFIG. 1 producing an “off” state in the modulator;

FIG. 5, is a graph of Group Delay vs. Frequency at an off-state for themodulator of FIG. 1;

FIG. 6, is a graph of Transmittance vs. Frequency at an on-state for themodulator of FIG. 1;

FIG. 7, is a graph of Phase vs. Frequency depicting the 2 π phasedifference between the lower arm and the upper arm of the modulator ofFIG. 1 producing an “on” state in the modulator;

FIG. 8, is a graph of Group Delay vs. Frequency at an on-state for themodulator of FIG. 1;

FIG. 9 is a schematic of a modulator constructed according to thepresent invention;

FIG. 10 is a pair of graphs showing the resonance overlap (10(a)) andrespective output intensity (10(b)) for the Mach-Zehnder modulatorconstructed according to the present invention where there exists lessthan substantial resonance overlap;

FIG. 11 is a pair of graphs showing the resonance overlap (11(a)) andrespective output intensity (11(b)) for the Mach-Zehnder modulatorconstructed according to the present invention, wherein there existssubstantial resonance overlap; and

FIG. 12 is a pair of graphs showing resonance the overlap (12(a)) andrespective output intensity (12(b)) for the Mach-Zehnder modulatorconstructed according to the present invention wherein there exists lessthan substantial overlap resulting from the continuation of displacementfrom FIG. 10 through FIG. 11.

DETAILED DESCRIPTION

The following merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the invention and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

FIG. 1A shows a schematic of a prior art optical modulator apparatuswhich is known to those skilled in the art as a Mach-Zehnder modulatorSuch optical modulators are one of the key components for signaltransmission systems and a number of types are known and understood. Ascan be appreciated, the simplicity of the Mach-Zehnder modulatorcontributes to its wide utilization in optical systems.

With continued reference to that FIG. 1A, it may be observed that theMach-Zehnder modulator structure includes an input waveguide 110 and anoutput waveguide 120, optically connected by a pair of waveguide arms130, and 140. Accordingly, an optical signal applied to the inputwaveguide 110 will exit the output waveguide 120 after traversing themodulator via upper arm 130 and/or lower arm 140.

Turning now to FIG. 1B, there is shown a Mach-Zehndermodulator—according to the present invention—that includes a pair ofring resonators 150, 160, wherein one ring resonator is positioned inboth the upper arm 130 and the lower arm 140, respectively.

At this point it should be noted that the resonator structures 150, 160are shown as being outside of the Mach-Zehnder structure. Those skilledin the art will appreciate that such resonators (either ring resonatorsor others) may be within the Mach-Zehnder structure(s) as well. For thepurposes of the present invention, it is only required that suchresonator structures be optically coupled to the arms of the modulator.

According to the present invention, the ring resonator 150 positionedwithin the upper arm 130 is set such that its resonance is offset fromlight to be modulated. Similarly, the ring resonator 160 positionedwithin the lower arm is set such that its resonance is offset to anopposite side of the modulated light.

Those skilled in the art will readily understand that the resonancepositions may be moved relative to one another and the light to bemodulated by any of a number of mechanisms. For example, thesemiconductor materials may be doped to create charge carriers, thenumber of charge carries may be changed, and the optical paths may bechanged through a redistribution of the charge carriers. Operationally,such resonance peak movement may be accomplished by applying a forwardbias to the structure or otherwise applying a necessary voltage over aparticular area of the semiconductor structure.

The spectral effects of these structural relationships may be understoodwith reference to FIG. 2, which shows a graph of optical power vs.frequency for the structure shown in FIG. 1B. More specifically, theresonance in one arm 210 and the resonance in the other arm 220 areshown graphically relative to the frequency range of the light to bemodulated 230.

According to the present invention, the modulator 100 of FIG. 1B is setto an “off” state when—as depicted in the graph of FIG. 3—the tworesonances of the upper arm and the lower arm sufficiently overlap eachother and the light to be modulated is “in-between” them.

As can be observed from this FIG. 3, the overlap between the tworesonances is only partial, consequently the Mach-Zehnder modulator isturned off predominately because of the phase changes caused by theresonance locations in each of the arms. It should be noted that whilewe have depicted the frequencies shown in the graph of FIG. 3 as beingassociated with a particular arm, i.e., “upper arm” or “lower arm”, thefrequencies shown associated with a particular arm are for the purposeof explanation only, and either of the arms may be associated with aparticular transmittance vs. frequency graph.

With reference now to FIG. 4, there it shows a graph of Phase vs.Frequency depicting the π phase difference between the lower arm and theupper arm of the modulator of FIG. 1, while FIG. 5 is a graph of groupdelay vs. frequency for the off-state of the modulator.

Conversely, the modulator is turned “on” by moving the resonances inopposite directions. Accordingly, if the resonances are moved orotherwise sharpened/made more narrow thereby reducing the overlapbetween them, this will effectively turn “on” the modulator. Such an“on” state is shown graphically in FIG. 6, which shows a graph ofTransmittance vs. Frequency for the modulator of FIG. 1 wherein theoverlap has been sufficiently reduced to produce the “on” state.

FIG. 7 is a graph of Phase vs. Frequency depicting the 2 π phasedifference between the lower arm and the upper arm of the modulator ofFIG. 1, while FIG. 8 is a graph of Group Delay vs. Frequency for theon-state of the modulator.

As can be appreciated, when the overlap between the two resonances inthe two arms is sufficiently reduced, the resulting phase shifts in eachof the arms of the modulator are in graphically-opposite directions.Advantageously, and according to the present invention, this produces anoptical modulation exhibiting a very small, or minimal amount of chirp.

Of further advantage, the overlap between the resonances in each of thearms may be tuned by changing the width, or position, or both—of theresonances. Turning our attention now to FIG. 9, there is shown aschematic of a modulator/resonator structure having a tunablecoupler—constructed according to an aspect of the present invention

More particularly, shown in FIG. 9 is a Mach-Zehnder (MZI) typemodulator 900, having input 910, and output 920 waveguides coupled toboth the upper 930 and lower 940 arms of the Mach-Zehnder structure. Asshown further in that FIG. 9, the upper arm 940 includes a resonatorring 950 having a ring phase shifter 960 positioned within the opticalpath of the ring. In addition, both upper 930 and lower 940 arms of theMZI structure include phase shifters 970, and 980, respectively.

When constructed in this manner, changing the ring phase shifter 960will predominately change the spectral location of the resonance. As canbe appreciated, when a change in the effective index of the waveguide isaccompanied by a change in the propagation loss of the waveguide, theaccompanying change in resonator loss will have an impact on theresonance shape.

Further, changing phase shifter 980 within the lower 940 arm of the MZIstructure, will change both the width and position of the resonance.Appropriately changing both phase shifters 970 and 980 willadvantageously change only the width (shape) of the resonance.

Additionally, note that if only one of the MZI phase shifters (970, 980)is changed, the symmetry of the associated optical phase changes in eacharm may be improved by setting the resonator in each arm appropriately.For example, according to the invention, the device may be drivenpush-pull so that the resonator in that arm which experiences anincrease in refractive index responds such that the resonance peakbecomes sharper. Likewise, the other resonator in that arm whichexperiences a decrease in index also responds such that the resonancepeak becomes sharper.

Finally, note also that a mixture of high-speed and low-speed tuningelements may be used to increase device efficiency and flexibility.

By way of demonstrating further examples of the present invention inoperation, differential phase shift keying (DPSK) and higher orderadvanced modulation formats advantageously benefit from the minimalchirp associated with the present invention. Accordingly, materials areemployed which exhibit a change in optical propagation loss when theoptical index is changed. Of particular interest, thisapproach—according to the present invention—may be employed to produceother amplitude-phase modulated formats in addition to DPSK includingduobinary, alternate mark inversion, alternate block inversion andothers.

As can now be appreciated, according to the invention, resonance peaksare tuned in opposition directions to minimize the chirp. In thispresent case however, the Mach-Zehnder modulator is configured such thatwhen the absorption peaks overlap each other the phase relationshipcreated from the two arms of the MZM results in a null output from theMZM. In preferred embodiments, the resonance responses from the two armsof the MZM exhibit a similar shape and each resonance peak is relativelysymmetric in its spectral absorption shape.

For example, with reference to FIG. 10, there it is shown the positionsof the resonances such that there is maximum output from the modulator.In these positions, there exists a constructive interference at theoutput waveguide of the MZM.

If the resonances are then shifted to those shown in FIG. 11, the outputfrom the MZM is minimized. Finally, if the resonances are furthershifted to those depicted in FIG. 12, the MZM output intensity isincreased to that shown where the phase is substantially equal to π.

Advantageously, and as noted before, depending upon the configurationand mode of drive, the output from the MZM may be a DPSK output or, ifthe response is appropriately bandwidth limited, the output may beduobinary.

At this point, while the present invention has been shown and describedusing some specific examples, those skilled in the art will recognizethat the teachings are not so limited. Accordingly, the invention shouldbe only limited by the scope of the claims attached hereto.

1. A semiconductor optical modulator comprising: a Mach-Zehnder deviceincluding at least two arms; and at least two optical resonators, eachone of said resonators being optically coupled to an arm of theMach-Zehnder device, wherein each of the arms is optically coupled to atleast one of the resonators; the semiconductor optical modulatorcharacterized in that: each of the resonators exhibit a particularresonance frequency such that when the resonance frequencies are tunedto substantially overlap one another the modulator is turned off andwhen tuned to less than the substantial overlap the modulator is turnedon.
 2. The optical modulator of claim 1 further comprising: a pluralityof phase shifters wherein at least one of said plurality of phaseshifters is positioned within an arm of the Mach Zehnder device foraltering the coupling ratio between the arm of the Mach-Zehnder deviceand the corresponding resonator, and at least one of said plurality ofphase shifters is positioned within one of the resonators for tuning theresonance frequency.
 3. The optical modulator of claim 1 furthercomprising: a plurality of phase shifters positioned within theresonators.
 4. The optical modulator of claim 1 further comprising: aplurality of phase shifters positioned within the arms of theMach-Zehnder device.
 5. The optical modulator of claim 1 wherein saidmodulator construction comprises materials selected from the groupconsisting of Silicon, Indium Phosphide, Gallium Arsenide, achromophore-doped polymer, a chromophore-based crystal, and thin-filmLithium-Niobate.
 6. A semiconductor optical modulator comprising: aMach-Zehnder device including two arms; and at least two opticalresonators optically coupled to the arms of the Mach-Zehnder device,wherein each of the arms of the Mach-Zehnder device is coupled to atleast one of the resonators; wherein each of the resonators exhibit aparticular resonance frequency; and a plurality of phase shifters,wherein at least one of said plurality of phase shifters is positionedwithin each arm of the Mach Zehnder device and alters the opticalcoupling ratio between each arm and the corresponding resonator, and atleast one of said plurality of phase shifters is positioned within eachone of the resonators tunes the respective resonance frequency, whereinupon simultaneously altering one or more of the coupling ratios andtuning the respective resonance frequencies, the resonance frequenciessubstantially overlap one another such that the modulator is turned toan ‘on state’ exhibiting a desired phase modulation.
 7. The opticalmodulator of claim 6 wherein the modulator operates in two ‘on states’exhibiting a ‘0’ or a ‘π’ phase shift between the ‘on states’, such thatthe modulator modulates an input signal according to a format selectedfrom the group consisting of differential phase shift keying (DPSK),differential quadrature phase shift keying (DQPSK), phase amplitudemodulation (PAM), duobinary, and their higher order embodiments.
 8. Amethod of operating a semiconductor optical modulator having: aMach-Zehnder structure including a pair of resonators, each resonatoroptically coupled to a respective arm of the Mach-Zehnder strucutre,wherein each one of said resonators exhibits a characteristic resonancepeak; the method comprising the steps of: altering the optical pathlength of one or more of the resonators such that the resonance peaksare brought closer together thereby imparting a desired phase shiftbetween optical signals traversing in the two arms of the modulator suchthat the modulator is turned to an ‘on state’; and altering the opticalpath length of one or more of the resonators such that the resonancepeaks are separated thereby imparting a desired phase shift between theoptical signals traversing the two arms of the modulator such that themodulator is turned to an ‘off state’.
 9. The method of claim 8 furtherincluding the steps of: altering the optical coupling ratio between theat least one arm and the corresponding resonator through the effect ofadjusting at least one phase shifter of a plurality of phase shifterspositioned in at least one arm of the Mach-Zehnder structure; andaltering the resonance peak through the effect of adjusting at least onephase shifter of said plurality of phase shifters is positioned within aresonator.
 10. The method of claim 8 wherein said modulator includes aplurality of phase shifters positioned in the resonators.
 11. The methodof claim 9 further including the steps of: operating the modulator intwo ‘on states’ exhibiting a ‘0’ or a ‘π’ phase shift between the ‘onstates’; and alternating between said two ‘on states’, such that saidmodulator is operated to modulate an input optical signal according to aformat selected from the group consisting of: differential phase shiftkeying (DPSK), differential phase shift keying (DPSK), differentialquadrature phase shift keying (DQPSK), phase amplitude modulation (PAM),duobinary, and their higher order embodiments.
 12. The method of claim11 method further comprising the step of: exchanging the positions ofthe resonance peaks.
 13. The method of claim 11 method furthercomprising the step of: altering the optical path length of one or moreof the resonators such that the positions of the resonance peaks areinterchanged.
 14. The method of claim 8 wherein said modulator includesmaterials selected from the group consisting of Silicon, IndiumPhosphide, Gallium Arsenide, a chromophore-doped polymer, achromophore-based crystal, and thin-film Lithium-Niobate.
 15. Asemiconductor optical modulator comprising: means for splitting anoptical signal into at least two subsignals; means for generatingresonant frequencies for each of the subsignals; means for producingsubstantial overlap of the resonant frequencies thereby turning themodulator off and means for producing less than the substantial overlapthereby turning the modulator on.
 16. The optical modulator of claim 15further comprising: means for shifting the phase of the subsignals. 17.The optical modulator of claim 16 wherein said shifting means isdisposed within said resonant frequency generating means.
 18. Theoptical modulator of claim 1, wherein the respective resonancefrequencies of the resonators are positioned on opposite sides of anoptical signal frequency traversing the modulator.
 19. The opticalmodulator of claim 1, wherein the resonance frequencies are tuned tomove in opposite directions relative to each other.
 20. The method ofclaim 8, wherein the optical path length of one or more of theresonators is altered by moving the resonance peaks in oppositedirections relative to each other.